As a result of the rapid progress in the development of high-power diode laser technology in recent years, many new applications for diode lasers have been found. These include, for example, the pumping of solid-state lasers by means of laser diodes.
A special feature of diode lasers is the asymmetric cross section of the exiting beam, which emerges from the emitting zone or exit surface. This beam has a characteristic elliptical form in cross section. The major axis of this elliptical cross section is perpendicular to the pn transition region of the diode structure (also called the "fast direction"), whereas the minor axis is parallel to the plane of the pn transition region (also called the "slow direction"). In addition to this asymmetric beam cross section, it is also true that the emerging radiation shows considerable divergence in the direction of the major axis, that is, in the direction perpendicular to the plane of the pn transition region or the active layer. The beam angle can be as much as 90.degree., whereas this divergence is only about 10.degree. in the direction of the minor axis.
Because of the special elliptical cross section of the beam, the wide divergence perpendicular to the active plane (also called the "junction plane"), and the relatively small divergence perpendicular to that, the use of diode lasers of this type depends significantly on how well the diode laser radiation can be guided and shaped, especially when many of these laser diodes are combined into laser diode fields or arrays.
The conventional method for guiding and forming the laser diode radiation involves the use of systems of transmitting optical components such as lenses and prisms, which are inserted into the beam path of the radiation emerging from the exit opening of the laser diode. In certain applications, such as in the case of the diode pumping of solid-state lasers, the diode radiation of each diode must be focussed by means of a lens on the solid rod. The problem here is finding a lens with an extremely high numerical aperture, by means of which all of the diode radiation with the above-described divergence angle of approximately 90.degree. in the direction perpendicular to the plane of the pn junction can be collected. In principle, such lenses can be made only of glass with a very high index of refraction. A disadvantage, however, is that the large angle of incidence causes a large amount of energy to be lost by reflection in the lens; this loss is typically more than 20%, which therefore decreases the high efficiency which, among other things, characterizes a diode laser.
High-power laser diodes typically have active media with a cross section of 1.times.100 .mu.m. For cost reasons, furthermore, several laser diodes are arranged with their beam exit surfaces in a row or in complex fields or arrays. So that laser diodes can produce a radiation field in cases where they are lined up in a row, the diodes are set up so that the major axes of the elliptical cross sections of the beams are all parallel to each other. Because the beam quality is diffraction-limited in the narrow direction and diffraction-limited by a factor of approximately 1,000 in the junction plane, the radiation emitted by a laser diode array cannot be focussed by cylindrical lenses and spherical lenses or by combinations thereof into a small, circular spot. This restricts the use of laser diode arrays in applications such as guiding the beam into an optical fiber or the so-called "end-on-pumping" of solid-state lasers.
On the basis of the problems discussed above, the task of the present invention is to provide an arrangement which can be used for laser diodes lined up in a row and on a common plane and by means of which the beams of the individual laser diodes can be mapped to an essentially uniform beam field or beam field pattern.